Meetings: [[0,30], [5,10], [15,20]]. Events: (,+1), (,+1), (,-1), (,+1), (,-1), (,-1). Sorted: (,+1), (,+1), (,-1), (,+1), (,-1), (,-1). Running count: -> -> -> -> -> -> . Maximum: . So rooms are needed.
The peak happens when meetings [0,30] and [5,10] overlap, and again when [0,30] and [15,20] overlap. The count tracks how many meetings are active at each event time. The maximum count is your answer. This approach simulates the timeline.